How to set a specific edge length numerically?
This feature is now in an included addon in Blender 2.79 called Mesh: Edit Tools 2. You have to enable the addon in the User Preferences.




Go to Add-ons
Search "edit"
Click the box for Mesh: Edit Tools 2
Save User Settings




While in Edit Mode, press W for the Specials Menu


Edit Tools > Set Edge Length
Input a target length and click OK.


Hope this helps! I searched forever to find the answer. Shoutout to Danyang Yi for providing the link to a previous version of this addon prior to it being included in Blender.